The considerable growth of the polyurethane industry has seen an equally considerable increase in the problem of eliminating and reusing polyurethane waste and polyurethane rejects. Although a market has been found for chips of soft polyurethane foam waste by bonding together the chips to form composite materials, it is nevertheless only possible in this way to use a limited quantity of soft-foam material. Unfortunately, there are no similar opportunities for utilizing waste of semi-hard and hard polyurethane foams or elastomer granulates. Accordingly, large quantities of polyurethane waste and rejects from the manufacture of hardened soft foams and elastomers must be dumped in waste collecting areas or destroyed in incinerators. Unfortunately, this gives rise to considerable ecological, technical and economic problems because of the low specific gravity and, hence, large volume of the waste and rejects. Accordingly, both for ecological and economical reasons, there is considerable interest in economically recycling the steadily increasing quantities of polyurethane waste.
German Offenlegungsschriften Nos. 2,362,919; 2,362,920 and 2,362,921 relate to processes for hydrolyzing polyurethane foam waste with steam at high temperatures for material salvage purposes. Unfortunately, these processes require high temperatures and pressures (for example 240.degree. C. and 40 atms), so that splitting up the polyurethane waste in this way can only be achieved at considerable expense and with an equally considerable outlay on apparatus. In addition, the reaction products accumulate in admixture with water so that they have to be separated off by special processes before they can be reused.
It is also known (U.S. Pat. No. 3,738,946) that polyurethane foam waste can be degraded into refoamable polyols by heating to 175.degree.-250.degree. C. in high-boiling dihydroxy compounds, preferably diethylene glycol, and advantageously in the presence of approximately 10% of a diethanolamine. In this way, it is possible by trans-urethanization to obtain short-chain polyols which, however, are only advantageously used for the production of hard polyurethane foams. According to the teaching of the above-mentioned U.S. patent the process is only advantageously used for splitting up hard polyurethane foams. Another disadvantage of this process is that it takes several hours to complete the reaction. Accordingly, the process uses a considerable amount of energy and cannot be readily worked in continuous installations.
It is known from German Offenlegungsschrift No. 2,238,667 that polyurethane foams can be linearized by impregnating the foam particles with lactams or lactam addition compounds in a matrix reaction. In this reaction, the chain-branching bonds (except for the isocyanurate group) are selectively split into polyurethane plastics, such as biuret, allophanate, uretonimine and uretdione groups. This reference clearly indicates, however, that chain-extending bonds, such as urethane and urea groups, and the isocyanurate bonds, are not affected.
In Agnew, Makrom Chemie 37, 59-88 (1974), it is noted that, according to exhaustive tests on low molecular weight test substances in the temperature range from 20.degree. to 130.degree. C., lactam associates split the above-mentioned chain-branching bonds very quickly, whereas they do not split urethane and urea bonds at all.
British Patent No. 1,308,468 describes a number of so-called matrix reactions on polyurethane foams. For example, it is disclosed that .epsilon.-caprolactam can be polymerized in a foam matrix (cf. Example 10 of British Patent No. 1,308,468), apparently without the polyurethane bonds being destroyed.